The incomplete combustion of organic materials, such as petroleum-based fuels, can result in the production of carbon containing particulates or soot. The release of these particulates, along with other combustion products, to the environment can lead to a variety of pollution problems. A number of ceramic based or other high temperature filter devices have been proposed for the purpose of removing soot from combustion gases. Once the filter has collected a certain quantity of soot, the pressure drop across the filter becomes excessive. At that point, the filter element must be either replaced or regenerated by the incineration of the soot in order to allow the filter to be returned to service. One of the more common regenerative methods is the addition of energy to the soot and/or filter to produce heat in order to promote combustion of the soot.
Although the invention described herein can be applied equally well to a variety of soot-filtration and filter-regeneration requirements, a particular application of interest is the elimination of soot generated by compression ignition of diesel engines. Diesel soot does not undergo significant oxidation at temperatures below approximately 400.degree. C. For many diesel engine applications, the average exhaust gas temperature is considerably below this temperature. Under these conditions diesel soot will continue to accumulate in a filter leading to filter blockage and unacceptable engine performance.
Diesel exhaust temperatures can be raised to 500.degree. C. to 700.degree. C. to induce filter regeneration by throttling of the engine. However, this type of regeneration necessitates operator intervention and suspension of normal engine operation for a period of time. For these reasons, throttling has not been widely adopted as a suitable method of filter regeneration. Alternately, external heat sources, such as flames or resistance heating, have been proposed to raise the soot to the required combustion temperature. These methods are either unreliable in initiating soot ignition or produce uneven heating of the filter, leading to either incomplete filter regeneration or destruction of the filter due to localized thermal stresses.
It is known to employ microwave energy to incinerate soot in the exhaust of diesel engines. Erdmannsdorfer et al. United Kingdom Published Application No. 2 080 140, published on Feb. 3, 1982, discloses an apparatus for removing soot from exhaust gases comprising an annular filter element, made of ceramic fibres, mounted on a perforated metal wall and concentrically disposed in a cylindrical resonant microwave cavity. Exhaust gases flow generally axially through the cavity but radially inwardly of the filter element so that particulates tend to accumulate on the outer surface of the filter element. Incineration of the soot is achieved by direct coupling of the soot particles with the microwaves. Since diesel soot is itself a lossy dielectric material, it absorbs energy from the electric component of the electromagnetic field. The electromagnetic field formed in the cavity is not axisymmetrically disposed about the filter element and, therefore, the device does not take full advantage of the electric field component of the microwaves. The patent does not describe any way of extracting heat from the magnetic component of the energy of the microwaves.
Puschner et al. U.S. Pat. No. 4,825,651 issued on May 2, 1989, discloses an apparatus which employs a tubular dielectric insert to concentrate the exhaust flow in an area of a cylindrical resonant cavity of highest energy density of the electromagnetic field produced by a microwave source. The soot is incinerated in the gas phase as it passes through the resonant cavity. Unlike Erdmannsdorfer et al, Puschner does not employ a filter element to trap soot. The patent does not disclose any mechanism which makes use of the energy of the magnetic field component of the microwaves.
Puschner et al. West German Patent No. 35 284 45 discloses direct microwave incineration of the soot augmented by microwave heating of a filter made of lossy dielectric material or a filter in close contact with a lossy dielectric insert. The soot is incinerated by indirect heating, i.e. by heating the lossy dielectric material, which then heats the soot. As mentioned above, diesel soot is itself a lossy dielectric material which absorbs energy from the electric component of the electromagnetic field. Hence, the incorporation of a dielectric material as proposed by West German Patent No. 3,528,445 does not provide a significant advance in the art because a dielectric material, in the form of soot, is already present. Further, like Erdmannsdorfer et al, this patent relies strictly of the electrical content of the microwaves and also fails to provide a mechanism of taking advantage of the magnetic energy content of the microwaves.
In summary, the state of the art relating to diesel filter regeneration using microwave technology is limited to the use of only the electric field component of the microwaves and does not disclose any mechanism for using the magnetic field component of the microwaves. The art has not appreciated the benefits of providing an axisymmetrically distributed, standing electromagnetic waves within the cavity so as to take full advantage of the energy of electric field component, let alone the magnetic field component. As a consequence, the regeneration processes of the current state of the art tend to be inefficient, if not incomplete and unsatisfactory.